Re: [PATCH v38 21/24] x86/vdso: Implement a vDSO for Intel SGX enclave call

[H.J. Lu]

On Mon, Sep 28, 2020 at 9:44 AM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>
> On 28/09/2020 01:58, Jarkko Sakkinen wrote:
> > On Fri, Sep 25, 2020 at 07:23:59PM +0100, Andrew Cooper wrote:
> >> On 15/09/2020 12:28, Jarkko Sakkinen wrote:
> >>> diff --git a/arch/x86/entry/vdso/vsgx_enter_enclave.S b/arch/x86/entry/vdso/vsgx_enter_enclave.S
> >>> new file mode 100644
> >>> index 000000000000..adbd59d41517
> >>> --- /dev/null
> >>> +++ b/arch/x86/entry/vdso/vsgx_enter_enclave.S
> >>> @@ -0,0 +1,157 @@
> >>> +SYM_FUNC_START(__vdso_sgx_enter_enclave)
> >>> <snip>
> >>> +.Lretpoline:
> >>> +   call    2f
> >>> +1: pause
> >>> +   lfence
> >>> +   jmp     1b
> >>> +2: mov     %rax, (%rsp)
> >>> +   ret
> >> I hate to throw further spanners in the work, but this is not compatible
> >> with CET, and the user shadow stack work in progress.
> > CET goes beyond my expertise. Can you describe, at least rudimentary,
> > how this code is not compatible?
>
> CET Shadow Stacks detect attacks which modify the return address on the
> stack.
>
> Retpoline *is* a ROP gadget.  It really does modify the return address
> on the stack, even if its purpose is defensive (vs Spectre v2) rather
> than malicious.
>
> >> Whichever of these two large series lands first is going to inflict
> >> fixing this problem on the other.
> >>
> >> As the vdso text is global (to a first approximation), it must not be a
> >> retpoline if any other process is liable to want to use CET-SS.
> > Why is that?
>
> Because when CET-SS is enabled, the ret will suffer a #CP exception
> (return address on the stack not matching the one recorded in the shadow
> stack), which I presume/hope is wired into SIGSEGV.
>

Here is the CET compatible retpoline:

endbr64
/* Check if shadow stack is in use.  NB: R11 is the only usable
   scratch register for function calls.  */
xorl %r11d, %r11d
rdsspq %r11
testq %r11, %r11
jnz 3f
call 2f
1:
pause
lfence
jmp 1b
2:
mov %rax, (%rsp)
ret
3:
/* Shadow stack is in use.  Make the indirect call.  */
call *%rax
ret


-- 
H.J.